Semiconductor materials are useful for making a wide variety of electrical and electro-optic devices. This is because of the band structure of the semiconductor material which makes up the device. Semiconductor materials have a conduction band and a valence band which make up the band structure of the material. The conduction band is a range of energy states for charge carriers (electrons) wherein a charge carrier existing in an energy state above a minimum value has the ability to quickly move around the material and conduct current. The valence band is a range of energy states for charge carriers (holes) wherein a carrier existing in an energy state below a maximum value has the ability to quickly move around the material and conduct charge. The minimum and maximum values are called the conduction band edge and the valence band edge respectively. The minimum conduction band edge is always greater than the maximum valence band edge in a semiconductor. The difference in energy between the conduction band edge and the valence band edge is called the bandgap. When carriers which have enough energy to be in the conduction band lose energy and make transitions between the conduction and the valence bands, the carriers give off light. The frequency of light emitted from the semiconductor device is proportional to the size of the bandgap. Therefore, the lost energy of the carriers can be converted into light having a specific frequency by tailoring the bandgap of the semiconductor material.
In order to have carriers lose energy and make a transition from a conduction band to a valence band, carriers must first exist in the conduction band. One way in which semiconductor devices act as a switch in conducting current is to inject carriers from the conduction and valence bands of one type of semiconductor material into the conduction and valence bands of a second type of semiconductor material. That is, electrons are injected from the conduction band of a n-type material to the conduction band of a p-type material by applying a positive voltage to the p-type material with respect to the n-type material across the junction of the two types of material. Similarly, holes are injected from the valence band of the p-type material to the valence band of the n-type material by applying a positive voltage to the p-type material with respect to the n-type material across the junction of the two types of material. As a result, forward biasing a pn junction semiconductor device places electrons in the conduction band of the p-type material and holes in the valence band of the n-type material. These carriers are available to make the bandgap transition and emit light having a frequency proportional to the bandgap energy.
The light emitting pn junction is useful because light is only emitted when the pn junction is forward biased, and therefore, an electrical signal in the form of a high or low voltage can be easily converted into an optical signal in the form of light or no light. An optical signal is useful because many materials have a distinct reaction to light and make optical storage devices which have a higher storage density than electrical storage devices. Further, optical communication is highly desirable because much more data can be transmitted over a single optical fiber than over an electrical connection. In either optical storage or optical communication, a means such as an LED for converting electrical signals to optical signals is a requirement for taking advantage of different optical devices. Prior art LEDs are made of a Group III-V compound such as GaAs.sub.1-x Al.sub.x wherein x is a mole fraction of aluminum and typically ranges between 0 and 0.5. LEDs are made of this compound because the GaAs.sub.1-x Al.sub.x system is relatively easy to dope and because the bandgap can be tailored to some degree. The GaAs.sub. 1-x Al.sub.x semiconductor material is easy to dope because it is relatively free of defects which trap the carriers and make the carriers from the dopant immobile. The bandgap can be tailored to some degree by increasing the amount of aluminum added to the III-V compound. As x is increased the bandgap increases to a maximum of approximately 2.2 eV.
It is important to tailor the bandgap of the semiconductor material because as the bandgap gets greater, the energy of the light emitted gets greater and the wavelength of the light gets shorter. The shorter the wavelength, the more signals which can be transmitted and the more data which can be stored in an optical storage media. The problem with the GaAs.sub.1-x Al.sub.x semiconductor compound is that the maximum bandgap which can be developed in the material is approximately 2.2 eV (at room temperature). This bandgap corresponds to a yellow emitted light. It would be desirable to have a semiconductor material with a higher bandgap capable of emitting shorter wavelength light. The prior art has recognized this and attempted to use different compound materials which can be both appropriately doped and give the proper bandgap. One such attempt is to use group II-VI compounds for the wide bandgap. These compounds such as ZnSe have bandgaps of approximately 2.7 eV.
The problem with such compounds is that they are not readily made into pn junctions. For example, ZnSe can be easily doped n-type but not p-type, and ZnTe can be easily doped p-type but not n-type. The problem is thought to be due to a self-compensation effect in which acceptor (donor) impurities are electrically compensated for by the creation of oppositely charged point defects. This results in the effective cancellation of the acceptor (donor) dopant. This effect is material dependent and especially pronounced in wide bandgap semiconductors. As a result, doping typically only works for one type of electrical conduction (i.e. p type doping for ZnTe or n type for ZnSe) even though some p type ZnSe devices have been demonstrated. Therefore, pn junctions in any one type of wide bandgap material are difficult to make.
One way in which to avoid the self-compensation effect is to form a pn hetero-junction. In such a structure, the choice of n and p type materials is based on the ease of doping the materials and the compatibility of different lattice constants, among other considerations. The region where radiative recombination takes place is determined by the relative amount of injected carriers from one side to the other (i.e. electrons from the n to p side or holes from the p to n side). The relative injection of carriers is controlled by the size of the band offset and the carrier concentration in each side of the pn junction. It is also important to have close lattice constants between the two materials so that dislocations and defects in the lattice will not prevent proper injection of carriers across the pn junction. Hetrojunctions made of p-type ZnTe and n-type ZnSe satisfy the doping consideration. However, these materials have approximately a 7% lattice mismatch when combined in a pn junction. This large mismatch generates defects in the pn junction and reduces the carrier injection across the pn junction. Another problem with this structure is the band offset. The conduction band offset is smaller than the valence band offset. This allows substantially more electrons to be injected into the p-type material than holes injected into the n-type material. As a result, more electron recombination in the p-type region will be generated than hole recombination in the n-type region. This will produce a lower energy and longer wavelength emission because the p-type ZnTe has a lower bandgap than n-type ZnSe.
Another prior art attempt to create a wide bandgap material employed a modulation doping technique to a short period, strained superlattice structure (SLS) in the ZnTe/ZnSe system. The technique demonstrated that p type conduction could be attained by doping the ZnTe layers p type with antimony (Sb) and interleaving the ZnTe layers with ZnSe layers. One period of the p type material has a 1 nm ZnTe layer and a 1 nm ZnSe layer and the p type layer has 300 such periods. The problem with this structure is that the hole concentrations is not high. Particularly, it is only approximately 10.sup.-3 /cm.sup.3, whereas practical device applications require hole concentrations of approximately 10.sup.17 /cm.sup.3. The low hole concentration means that the dominant radiative recombination will be electrons recombining in the lower bandgap p-type material. Moreover, band to band emission was not the dominant radiative recombination process because the intensity of photoluminescence peaked at approximately 2.006 eV which is much lower than normal band to band recombination.